The Bomb, Medium Capacity, (Grand Slam) was a earthquake bomb used by RAF Bomber Command against German targets towards the end of the Second World War. The bomb was originally called Tallboy Large until the term Tallboy got into the press and the code name was replaced by "Grand Slam". The bomb was similar to a large version of the Tallboy bomb but a new design and closer to the size that its inventor, Barnes Wallis, had envisaged when he developed the idea of an earthquake bomb. It was the largest and most powerful conventional (non-nuclear) aerial bomb used by either side during the war.
Medium Capacity (MC) bombs were designed to remedy the shortcomings of General Purpose (GP) bombs, with a greater blast and casings which were robust enough to confer a considerable capacity to penetrate, especially the Tallboy and Grand Slam. The Grand Slam case was made of a chrome-molybdenum alloy steel, with the charge constituting over 43 per cent of the bomb's weight.
Standard Avro Lancaster bombers could not carry the bomb; Avro built 32 Lancaster B.Mk 1 (Special)s with more powerful Merlin engine variants, a stronger undercarriage, without bomb bay doors and minus many items to save weight. When airborne with the Grand Slam, a Special could barely manoeuvre and pilots were advised to refrain from minor adjustments of the flying controls, allowing the aircraft to wallow. A Lancaster which returned with its bomb was not permitted to land at RAF Woodhall Spa, the 617 Squadron base; it had to use the longer runway at RAF Carnaby.
From 14 March to 19 April 1945, 42 Grand Slams were dropped on Germany. When landing on reinforced concrete, the bombs tended to break up or explode prematurely. The bombs had been designed to land in soft ground, penetrate deeply and then explode, creating a , causing the structure above to subside. Grand Slams were the largest bombs by explosive yield used by the Allies until the advent of atomic weapons later that year but remained the largest by weight until the deployment of the T-12 Cloudmaker in 1949. After the war, the Royal Air Force and United States Army Air Forces used Grand Slams and other bombs for research.
Background
Medium Capacity bombs
Medium Capacity (MC) bombs were designed to overcome the shortcomings of general-purpose bombs (GP bombs) whose explosive content was about 27 per cent of their weight (in contemporary German bombs the explosive weight was 50 per cent). MC bombs were to have an explosive content of 40 per cent by weight and use explosives of greater power, although shortages often led to inferior explosive types being used. High Capacity (HC) bombs had an explosive content of up to 75 per cent. MC bombs had greater blast effect than GP bombs and had casings which were robust enough to confer a considerable capacity to penetrate.
Grand Slam
On 18 July 1943, after the Dams raid, the Controller of Research and Development at the Ministry of Aircraft Production (MAP) received a requirement for twelve Tallboy Small [] bombs, 60 unfilled Tallboy Medium (Tallboy) bombs and 100 Tallboy Large. Lack of capacity led to work on the Tallboy Large being stopped on 30 September. Interest in the Tallboy Large, renamed Grand Slam on 22 November 1944, after the term Tallboy had appeared in the press, increased after better understanding of the effect of the Tallboy was gained by the inspection of targets in Europe. The Tallboy was discovered to be too small to destroy concrete buildings with direct hits; only near misses which opened a under the foundations were found to be effective. In September 1944 the German V-2 rocket bombardment of London began, which had been expected by the British since Operation Hydra (the attack on the Peenemünde research centre on the night of 17/18 August 1943) and Grand Slams were considered as a weapon to be used on V-2 launch sites.
Doubts remained at the Air Ministry where an officer in the planning department was sceptical, even with the discovery of the limits of the Tallboy bomb, about what targets the Grand Slam could be dropped on. He also questioned whether, when ready in February–March 1945, there would be anything left to bomb and that aircraft adapted to carry the Grand Slam could not carry other loads. Air Commodore Christopher Bilney replied that the Tallboy Large would be more useful than the Upkeep mine used in the Dams raid. At the Bomber Command headquarters in High Wycombe, there was a debate about how many Tallboy Larges to order, 25 to 75 per month being proposed. The head of Bomber Command, Air Marshal Sir Arthur Harris, thought that the new bomb might be useful against U-boat pens and the bigger targets of Operation Crossbow (anti-V-weapons attacks) but put the burden on the Air Ministry to decide how many to manufacture. In September the decision to go ahead was made and 600 were ordered in October; 200 from British sources and 400 from the US. The British order was cancelled on 4 October, leaving only US production but RAF representatives in the US were able to persuade the Americans to build fifty bomb casings by the end of the year, to be filled in Britain. Four Lancaster bombers were ordered for trials and then reduced to one aircraft. During the Battle of the Bulge (16 December 1944 – 25 January 1945) any complacency about an imminent end to the war was confounded; the Grand Slam production order was revised again for 25 from British sources and 200 from the US.
An expanded version of the Tallboy design was not feasible and the English Steel Corporation of Sheffield began work on "large boilers" in late 1944. Experiments with the composition of the bomb casing, a steel alloy of chromium–molybdenum, were undertaken to find one that could withstand the shock of impact. A concrete core, accurate to 1/10,000th of an inch was made, then a sand-covered outer mould was created, into which the molten metal was poured. The core was chipped out after the metal had cooled but hot places on the casing could result in structural flaws, breaking the bomb casing when it hit, with no guarantee of the detonation of its contents. The US casings were made by electric resistance welding but both types had to pass stress tests to weed out inadequate examples. The British-made casings were sent on specially built road trailers to be machined, the nose on one and the tail on another.
The nose sections were filled by upending them to pour in of Torpex explosive a bucketful at a time. Once the tail section was connected, the Grand Slam was long, at its widest diameter and weighed . As with the Tallboy, the fins of the Grand Slam were angled, generating a stabilising spin and the Grand Slam had a charge-to-weight ratio of 43 per cent. Four fuzes were provided, one specially made for the Grand Slam with an 11-second delay, the No. 53 with a thirty-minute delay, a No. 53A with an hour's delay and a No. 37 which had a delay of 6 to 144 hours. The first bomb arrived at Woodhall Spa on 20 January and fifteen were expected by the beginning of February.
Lancaster B.Mk 1 (Special)
The ten remaining Dambuster Lancasters were sent for storage at the end of 1944 and 617 Squadron was increased from two flights to three, at the expense of 51 Squadron, a Halifax squadron of 4 Group, which lost a flight. To carry the Grand Slam, specially modified Lancasters were necessary but these would have to be subtracted from the production of ordinary Lancasters, when several 5 Group Halifax squadrons were converting to Lancasters. The weight of the Grand Slam meant that only new aircraft would be able to carry it, rather than converting existing and worn aircraft; 32 B.Mk 1 (Special)s were ordered from Avro. There was little agreement on what equipment the specials should carry and Avro backslid on production to keep the new Avro Lincoln, a Lancaster derivative, on schedule. Work began on the specials at the end of January 1945 and two had been built by 13 February, with twelve due within ten days and 24 by month's end, the remainder to be delivered in March.
PB995 was sent to the Aeroplane and Armament Experimental Establishment (A&AEE) with PB592/G, a conventional B.Mk 1 built in 1944, which was to be modified to a B.MK 1 (Special). Some equipment did not need to be fitted but other items were removed, which began a period of removals and reinstatements; a conference on 25 February was necessary to sort out the confusion. The front and mid-upper turrets were to go, along with the H2S, its fairing and the TR1196 R/T set, electric lights, flare chutes, flame dampers, some of the fuel tanks, the rear turret armour, three fire axes, a toolkit and the crew ladder. The commander of 5 Group, Air Vice-Marshal Ralph Cochrane ordered that the Identification friend or foe (IFF) be got rid of in favour of the VHF Bomber Fixer. Despite bombing by day, navigation equipment was needed because of the north-west European climate; GEE and LORAN were to be carried along with high-altitude and low altitude radio altimeters. The recognition lights and some of the usual VHF equipment were to stay along with the rear guns and 6,000 rounds of ammunition. The bomb doors had to go as even the widened Tallboy bomb doors would not fit a Grand Slam and as the mid-upper gunner and wireless operator were redundant, the crew was cut to five men.
While carrying a Grand Slam, a Lancaster would be incapable of taking evasive action and Fishpond tail-warning radar went with the H2S set. As Messerschmitt Me 262 jet fighters were operating in northern Germany, an Automatic Gun-Laying Turret (AGLT; also, Village Inn or Z Equipment) was considered. The AGLT rear turret had been under test by the Bomber Development Unit and two Main Force squadrons of 1 Group, which found that they restricted the view of the rear-gunner; the radar gun laying device was also unable to discriminate between German fighters and friendly bombers. Bombers equipped with the turret only flew when not in the company of other Allied aircraft. The aircraft of several bomber groups had been fitted with AGLT transmitters which worked best as an early-warning device and the B.Mk.1 (Special)s were fitted with AGLT in the nose. With a fuel load of the Special would weigh and the first was delivered to RAF Woodhall Spa, the base of 617 Squadron in Lincolnshire, on 25 February. There were still arguments about what the Specials would carry, when it was found that Avro had failed to install GEE sets. It was agreed to remove the front turret and its accoutrements; heating was needed for the rear guns and the Stabilised Automatic Bomb Sight had Tallboy dropping equipment but no capacity to drop practice bombs, which needed to be incorporated. The TR1154 W/T was to be replaced by VHF R/T.
Prelude
B.Mk1 (Special) in flight In March, Lancaster wheels and tyres were replaced by those of the heavier Avro Lincoln and the maximum weight of the Special was set at with a maximum landing weight of excepting emergencies. The commander of 617 Squadron, John "Johnny" Fauquier, ordered that if returning with a bomb, the Special would divert from Woodhall Spa and use the longer runway at RAF Carnaby near the coast at Bridlington in East Yorkshire. The centre of gravity when loaded with a Tallboy or Grand Slam was within the aircraft design limits, provided the rear turret was retained. Merlin 24 engines [ at 3,000 rpm, at 3,000 rpm, + boost at ] were to equip the Special, fitted with paddle-bladed propellers. Particular attention was to be paid to the skin of the wings and rear fuselage when checking the airframe and the Special would take off only from smooth runways and always with experienced pilots. Fuel was to be used from inboard tanks to outboard ones to relieve the stresses on the wings,